Heating an underground reservoir by radioactivity to recover viscous and tarry deposits therefrom



United States Patent HEATING AN UNDERGROUND RESERVOIR BY RADIOACTIVITY T0 RECOVER VISCOUS AND TARRY DEPOSITS THEREFROM Milton Williams, Houston, Tex., assignor, by mesne assignments, to Esso Production Research Company, Houston, Tex., a corporation of Delaware N0 Drawing. Filed Dec. 16, 1960, Ser. No. 76,111

9 Claims. (Cl. 166-9) This invention relates to the recovery of petroleum deposits from earth formations, and more particularly to the heating of earth formations to mobilize petroleum deposits therein.

In various localities, vast deposits of extremely viscous petroleum deposits are known to exist. Typical of these deposits are the bituminous Athabasca tar sands in Alberta Province, Canada, and the viscous crudes of the San Ardo field in California. The oil or tar in the Athabasca sands generally have a specific gravity varying within the range of 1.00 to 1.04. The oil in the Athabasca sands is a viscous tarry material which is extremely diflicult to recover by normal producing operations. The viscosity of the in-place crude of the San Ardo field is in the range of 20,000300,000 Saybolt Universal seconds at 80 F. and has a pour point of about 55 F. The production problems encountered in producing such petroleum deposits are substantial, and are particularly so with regard to the Athabasca tar sands. To date, no economically feasible process has been developed for recovering the oil therein.

It has been determined that if the Athabasca tar sands are heated to about 200 F., the tar becomes sufficiently fluid to be produced. Various systems have been developed to heat the tar sands to the requisite temperature, but for one reason or another, these systems have not been adopted. Generally speaking, the systems all involve large expenditures of money for equipment and require high-cost heat sources which are not utilized with high efficiency.

In any system for recovery of bituminous tar sands, it is desirable that only petroleum be removed from the earth in view of the problems and costs associated with waste disposal.

Objects and features of the invention which are not apparent from the above discussion will become evident upon consideration of the following detailed description thereof.

In accordance with the teachings of the invention, an insoluble radioactive precipitate is formed in the petroleum deposit. The precipitate is such that it has an average radioactivity of at least one curie per cubic meter of the petroleum containnig earth formation, over a period of at least one year. Preferably, the average radioactivity per cubic meter is much greater than one curie, and typically is of the order of 100 to 500 curies per cubic meter. The average radiation required will vary, depending on the temperature rise desired, thickness of the reservoir, the areal extent and geometrical configuration of the reservoir, and the half life of the radioactive material. Reference is made to Publication 588 of the National Academy of SciencesNational Research Council (1958). Radioactive material as specified above will be found to heat a tar sand, such as the Athabasca tar sand, to a temperature of over 200 P. so that the petroleum is readily produced therefrom. When the petroleum is produced, the sand grains will form a very effective filter to prevent the radioactive material from being removed from the sand along with the petroleum so that petroleum uncontaminated by radioactive elements is available for further processing by conventional refining techniques.

3,233,66 Patented Feb. 8, 1966 Preferably, the insoluble radioactive material is deposited in the earth formation by pumping into the formation a clear solution of the radioactive element, and reacting the radioactive solution in situ to form a radioactive insolute. By clear solution is meant a solution in which there is no solid phase; in other words, one that contains no suspended solids therein. Such a solution will flow readily into the tar sand. Usually, the tar sands contain water present in the form of films surrounding individual sand grains and insulating the grains from the oil and tar which fills the interstices of the mass. The solution is readily injected into the tar sand, for example, by drilling a first cased and tubed borehole to the desired depth in the sand, packing off the borehole to isolate the formation, and pumping the solution through the tubing under heavy pressure so as to displace the water surrounding the sand grains. A second borehole may be drilled a convenient distance away from the first borehole so that water may be removed from the formation as the radioactive solution is being injected into the formation through the first borehole. Inasmuch as the tar sand seldom exceeds 1200 feet in depth, the drilling of any desired number of boreholes is relatively easy and presents no particular problem.

In the preferred embodiment of my invention, the radioactive solution comprises an aqueous solution of an alkali hexametaphosphate, preferably sodium hexametaphosphate, (Na P O into which is dissolved a salt, hydroxide, or oxide of a radioactive element selected from the group consisting of strontium 85, strontium 89, strontium 90, barium 140, calcium 41, calcium 45, cerium 141, cerium 144, and mixtures thereof. Of the above, it will be found that strontium 89 and strontium 90 are preferred. Alkali hexametaphosphates have the property of sequestering cerium, calcium, barium, and strontium to form a complex chemical compound wherein the cerium, calcium, barium, or strontium is presumably held by weak coordinate forces. These compounds, termed cerium polyphosphates, calcium polyphosphates, barium polyphosphates, and strontium polyphosphates, respectively, are quite soluble in water, and the solution is a clear solution as defined above. The formation of the clear solution is insured by using an excess amount of alkali hexametaphosphate in forming the solution. The cerium, calcium, strontium, and barium polyphosphate complexes slowly decompose to form cerium orthophosphate, calcium orthophosphate, strontium orthophosphate, and barium orthophosphate, respectively, all of which are insoluble in connate earth fluids. The rate of the decomposition varies with the temperature of solution. The decomposition will go forward at temperatures as low as 33 F. and will accelerate in rate in accordance with the normal chemical relaationship wherein reaction rate approximately doubles with each 10 C. rise in temperature.

While alkali hexametaphosphates are herein disclosed as preferred sequestering agents, other sequestering agents may be used, such as Versene (which is disodium ethylenediaminetetracetic acid) having a pH between 7 and 12. The insoluble decomposition products of Versene that has sequestered cerium, calcium, barium, or strontium are the ethylenediaminetetraacetates of cerium, calcium, bari-v um, or strontium.

Under certain circumstances, it is desirable that nonradioactive materials such as nonradioactive calcium, strontium, barium, and cerium be admixed with the radioactive materials described above, preferably in the form of hydroxides, salts, or oxides. Preferably the mixture is in the ratio of between and radioactive material to nonradioactive material to form a clear, radioactive solution. The reason for this procedure is to minimize the amount of radioacive material that will not precipitate from the solution in the manner described above. As-

aaaaeeo sume, for example, that 0.1% of soluble radioactive strontium compounds remain in the solution after a given length of time, when only radioactive strontium was used for the purpose of preparing the solution. When nonradioactive strontium also is used in preparing the solution, in the ratio of 100 parts of nonradioactive strontium to one part of strontium 90, the amount of soluble strontium compound 90 remaining in the solution after the same length of time is only 0.001%.

It is to be noted that certain of the radioactive materials discussed above are undesirable products of the generation of electrical power utilizing fissionable elements as the primary source of energy. The present invention therefore provides a technique for economically disposing of such undesirable byproducts.

A certain tar deposit was determined to have an average thickness of approximately 40 meters and to have a pore space available for injected solution equal to 10% of the bulk volume of the reservoir material. It was further determined that a temperature rise of 85 C. was necessary to raise the reservoir temperature to the point where the petroleum therein could be produced. A quantity of 40 mg. of strontium 90 per gallon of strontium 90 polyphosphate solution prepared as described above, with a concentration of 5 grams of sodium hexametaphosphate per gallon, has an activity of 6 curies per gallon. A total quantity of 25,000,000 barrels of solution per square kilometer is required to fill the pore space. At the end of ten years the reservoir will be heated to a temperature suflicient to begin production of oil.

While my invention has thus been described with reference to the foregoing specific details, it is to be understood that further modifications will now be apparent to those skilled in the art. The invention therefore should not be considered as limited to the specific details set forth, but its scope is properly to be ascertained by reference to the appended claims.

What is claimed is:

1. A method of heating a petroleum reservoir to mobilize petroleum deposits therein comprising the step of pumping into the reservoir a clear radioactive solution of limited temporal chemical stability that forms an insoluble radioactive decomposition product.

2. A method of heating an earth formation to mobilize petroleum deposits therein, comprising: injecting a clear, radioactive solution into the formation; and, in the formation, reacting the radioactive solution to form a radioactive insolute.

. 3. A method of heating an earth formation to mobilize petroleum deposits therein, comprising: injecting a clear, radioactive solution into the formation; and, in the formation, precipitating a radioactive insolute from the solution.

4. A method of heating an earth formation to mobilize petroleum deposits therein, comprising: in an aqueous solution of alkali hexametaphosphate, dissolving a radioactive material selected from the group consisting of stron tium salts, strontium oxides, strontium hydroxides, barium salts, barium oxides, barium hydroxides, calcium salts, calcium oxides, calcium hydroxides, cerium salts, cerium oxides, cerium hydroxides, and mixtures thereof, to form a clear radioactive solution; and injecting the solution into the earth formation.

5. A method of heating an earth formation to mobilize petroleum deposits therein, comprising: in an aqueous solution of alkali hexametaphosphate, dissolving a material selected from the group consisting of strontium salts,

strontium oxides, strontium hydroxides, barium salts, barium oxides, barium hydroxides, calcium salts, calcium oxides, calcium hydroxides, cerium salts, cerium oxides, cerium hydroxides, and mixtures thereof, in the ratio between and radioactive material to nonradioactive material to form a clear, radioactive solution; and injecting the solution into the earth formation.

6. A method for recovering viscous petroleum deposits from an earth formation, comprising: injecting into the formation radioactive strontium dissolved in a solution of alkali hexametaphosphate, and producing the formation only after a sufiicient time has elapsed for strontium orthophosphate to substantially entirely precipitate from the solution.

7. A method of producing viscous petroleum deposits from an earth formation, comprising: drilling at least first and second boreholes into the formation; injecting into the formation through the first borehole a clear, radioactive solution selected from the group consisting of cerium polyphosphate, calcium polyphosphate, strontium polyphosphate and barium polyphosphate, and removing connate earth fluids from the second borehole simultaneously therewith; and producing the formation through one of the boreholes only after a sufficient time has elapsed for substantially insoluble orthophosphate to be substantially entirely precipitated from the solution.

8. A method of heating an earth formation to mobilize petroleum deposits therein, comprising: injecting into the formation a clear solution of radioactive material selected from the group consisting of cerium polyphosphate, calcium polyphosphate, strontium polyphosphate, barium polyphosphate, and mixtures thereof; and producing the formation only after a sufficient time has elapsed for substantially insoluble orthophosphate to be substantially entirely precipitated from the solution.

9. A method of heating an earth formation to mobilize petroleum deposits therein, comprising: injecting into the formation a clear solution of limited temporal stability of a radioactive material selected from the group consisting of cerium, calcium, strontium, barium, and mixtures thereof in a liquid sequestering agent therefor; and in the formation, precipitating a radioactive insolute from the solution.

References Cited by the Examiner UNITED STATES PATENTS 2,108,783 2/1938 Smith 23106 2,246,726 6/1941 Garrison 16638 2,308,176 1/1943 Howell 1665 2,795,279 6/1957 'Sarapuu l6611 3,108,439 10/ 1963 Reynolds 166-42 OTHER REFERENCES Enright, Radioactive Wastes May Become Future Oil Recovery Tool, The Oil and Gas Journal, July 13, 1959, vol. 57, No. 29, pages 72 to 74.

McNiel, Three Ways To Improve Oil Recovery, The Oil and Gas Journal, January 19, 1959, vol. 57, No. 3, pp. 86 to 98.

JACOB L. NACKENOFF, Primary Examiner.

BENJAMIN BENDETT, NORMAN YUDKOFF,

CHARLES E. OCONNELL, BENJAMIN HERSH, Examiners. 

1. A METHOD OF HEATING A PETROLEUM RESERVOIR TO MOBILIZE PETROLEUM DEPOSITS THEREIN COMPRISING THE STEP OF PUMPING INTO THE RESERVOIR A CLEAR RADIOACTIVE SOLUTION OF LIMITED TEMPORAL CHEMICAL STABILITY THAT FORMS AN INSOLUBLE RADIOACTIVE DECOMPOSITION PRODUCT. 